This invention relates to a controllable normal force generating mechanism that can be used on an in-pipe inspection robot to control the friction between a robot wheel and a pipe wall to assure proper traction with a minimization of expended energy.
Pipe systems are perhaps the most important infrastructures in modern societies. It is very important to monitor such pipelines in order to ensure proper working conditions. However, most of the pipe networks are buried under the ground and consequently are not easily inspected by human operators. Hence, the deployment of robots inside pipes for inspection purposes is of great interest [1].
Different locomotion strategies are used for in-pipe robots. The most common method, among such strategies, employs wheels attached to legs [17]. Wheels provide efficient means of propulsion but require pressing/normal force to maintain traction with pipe walls. With only a few exceptions [2], almost all wheeled in-pipe robots [3]-[10] use a linkage mechanism similar to the one shown in FIG. 1 to provide this pressing force. This mechanism is effective but has a limitation of being passive. The amount of normal force exerted is determined by the stiffness of the spring and cannot be changed actively.
On the contrary being able to control the normal force applied on the wheel is highly desirable for a number of reasons. First, the required friction force to maintain traction varies in different situations. For example, an in-pipe robot traveling through a vertical section of a pipe will require far more friction that when moving horizontally due to its own weight. But without an ability to adjust the normal force, an in-pipe robot must maintain unnecessarily high friction when traveling through horizontal pipelines in order to climb the vertical sections. This leads to a significant energy loss because most of the pipelines consist of horizontal sections and motors driving the wheel will consume more energy in the presence of high friction. An active normal force generating mechanism can reduce such energy losses by providing just enough friction to keep traction through appropriate feedback control.
Furthermore, a friction varying mechanism provides effective means of controlling the speed of in-pipe robots for liquid pipe networks. Chatzigeorgiou et. al [1] suggest a robot inside a liquid pipe (e.g., water pipeline) that will naturally travel with the flow moving at the speed of the liquid. The active normal force generating mechanism can act as speed-regulating mechanism for such liquid pipe robots. By controlling the friction force with wheels locked, the robot can slow down to the desirable speed from that of liquid flow speed.
A few wheeled in-pipe robots can actively control the normal force [11]-[14] use a similar linkage mechanism to the passive ones. Instead of having a spring on a slider shaft, these robots have either a motor or a pneumatic actuator to press on the linkage. The normal force on the wheel is controlled by varying the force at which the actuator presses on the linkage. However, using a linkage mechanism to actively control the friction force is energy inefficient. It is clear that the higher the friction to be generated, the higher the motor torque required to press on the linkage. Thus, a large friction force can only be produced at an expense of large energy consumption from the motor in prior art designs.
In this patent application, a novel mechanism capable of generating controllable friction force over large ranges with very low torque requirements is presented. Low torque requirements is translated to minimal energy consumption. This in turn can greatly lengthen the operation time of an in-pipe robot.